This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/SE99/02008 which has an International filing date of Nov. 5, 1999, which designated the United States of America and was published in English.
The present invention relates to a method of purifying sewage in sewage systems which comprise a toilet by adding a precipitant for precipitating phosphates.
Leakage of phosphorus from sewage systems, particularly those that are not connected to municipal sewage systems, is a growing problem. The sludge separation in such systems as are not connected to municipal sewage systems is often formed as a two-compartment septic tank system or three-compartment septic tank system with an associated infiltration installation. The sewage system comprises at least one toilet and may also comprise outlets of washing machines, dishwashers and other sources of sewage.
It is previously known to precipitate phosphate salts from such sewage systems by means of precipitants in the form of metallic salts such as iron salts or aluminium salts. Conveniently, the salts are selected from the group consisting of sulphates, chlorides or nitrates. According to prior-art methods of precipitation, the precipitant is added to the sludge separator, for instance, by the precipitant either being provided in solid form in the sludge separator or dosed in liquid form in the sludge separator.
Such prior-art methods are associated with disadvantages. When providing a solid precipitant in the sludge separator, it can be dissolved and supplied to the outlet continuously, independently of the supply of sewage to the sludge separator, since the atmosphere in the sludge separator is saturated with water vapour that continuously can dissolve the precipitant. This implies an unnecessary high dosage of the precipitant, which entails a risk that the volume of the sludge in the sludge separator increases to such a degree that more frequent emptying intervals may be required. In time-controlled dosage of liquid precipitants, installation of electric current and of a dosing device in the sludge separator is required, which is expensive. Moreover, the time-controlled dosage occurs independently of the generation of sewage, which as in connection with the solid precipitant causes a risk of an unnecessarily high dosage of the precipitant and the volume of the sludge increasing so much that more frequent emptying intervals are required.
Consequently, there is a need for a method of precipitating phosphates and other pollutants in the sewage of sewage systems, particularly those not connected to municipal sewage treatment plants but comprising a sludge separator, for instance in the form of a two-compartment septic tank or three-compartment septic tank, and a toilet which is connected thereto, in which case the precipitation must be adapted to the supply of phosphates which is added to the sewage system.
According to the invention, the above objects are achieved by a method of purifying sewage in sewage systems which comprise a toilet by adding a precipitant, characterized in. that the precipitant is selected from the group consisting of iron salts and aluminum salts and is adapted to be flushed with water in the bowl when flushing.
Further features and advantages of the invention are apparent from the dependent claims and the following description.